1. Field of the Invention:
The present invention relates to a numerically controlled grinding machine capable of machining a workpiece according to numerical control data. More specifically, the invention relates to a numerically controlled grinding machine capable of machining a workpiece by a rough grinding operation and a finish grinding operation.
2. Description of the Prior Art:
In a numerically controlled grinding machine such as a numerically controlled internal cam grinder, two different grinding wheels, i.e. a rough grinding wheel and a finish grinding wheel, are used for the rough grinding operation and the finish grinding operation, respectively. In such a grinding machine, the rough grinding operation is accomplished in such a way that an unground portion equal to a predetermined finish allowance remains after rough grinding. Then, the unground portion is ground in the finish grinding operation in order to produce a predetermined final cam profile on the workpiece.
In such a numerically controlled internal cam grinder, however, only the finish grinding wheel is dressed in the successive machining operation. Namely, dressing for the rough grinding wheel is usually omitted so as to shorten the machining cycle time. Accordingly, the unground portion which remains after the rough grinding operation becomes larger than the finish allowance when the wear of the rough grinding wheel occurs. However, the finish grinding wheel is positioned according to numerical control data, which are programmed based upon the theoretical finish allowance, regardless of the amount of the unground portion after the rough grinding operation. As a result, the wheel spindle which supports the finish grinding wheel is bent by the grinding force which changes depending on the amount of the unground portion after the rough grinding operation. The bending of the wheel spindle causes the grinding resistance in the finish grinding operation to change, so that it becomes difficult to grind workpieces accurately in a short period of time.